The invention relates to a lighting device for installation in a substantially plane surface.
Lighting devices of the aforementioned type are particularly useful in airports, and especially for use as marking of the correct path for incoming aircraft or aircraft taxiing on the runway, or to make centre lines or limits visible to the pilots.
A number of lighting devices for installation in a plane surface, and in particular for use in airports, are previously known. A device of the type mentioned above is taught in U.S. Pat. No. 4,860,182. This device comprises a prism holder for installation in a plane, preferably horizontal surface, and in particular in a runway. The prism holder has two orifices in its upper surface. A light refracting means in the form of a prism grating is arranged in each orifice. Each prism grating consists of parallel glass discs having a common longitudinal direction and of uniform thickness that are bonded adhesively to one another.
The prism gratings preferably consist of borosilicate glass, and a metal foil may be placed between each disc. Each prism grating is secured in place in the prism holder by means of an adhesive.
Positioned in the lower part of the prism holder are two angle-set light sources, each having a reflector arranged to emit light in rays that are essentially parallel in a principal direction. Each light source and reflector is inclined in such a manner that the principal direction of the light source is parallel to the common longitudinal direction of the parallel discs in the associated prism grating. The inclination, expressed as the angle that the principal direction of the light sources forms with a vertical axis, is said to be 45°.
The end of the prism grating that faces the corresponding light source forms an incident face that is essentially at right angles to the principal direction of the light sources. The incident face may be plane, convex or concave. In the first-mentioned case, the parallel light beams from a light source fall essentially perpendicular to the plane incident face or plane of incidence. The prism grating also has a plane exit face, which is parallel to the surface in which the device is to be located. The light in the prism grating strikes the plane of reflection at an angle that is not a right angle. Therefore, the light will be refracted so that it is transmitted at an angle of reflection determined by the refraction properties of the prism grating and the aforementioned angle of 45°.
When installed and used in a runway, the prism holder is inserted into a recess in the runway, with a resilient layer between the bottom of the recess and the prism holder. The top face of the prism holder has projections on either side of each of the prism grating orifices. The height of the projections corresponds to the thickness of the resilient layer. If a sufficiently large force is exerted on the top face of the prism holder, for example, by an aircraft passing with one wheel directly on the prism holder, the layer will yield elastically so that the prism holder temporarily moves slightly downwards in the recess.
A first disadvantage of this known device is that the projections in use form irregularities or bumps for passing aircraft and other vehicles. In particular, the projections are an obstacle for snow clearing in the winter season.
A second disadvantage of the known device consists of the weaknesses associated with the elastic flexibility of the prism holder in the vertical direction. This flexibility causes operational problems over time, due in part to the fact that the properties of resilient layer between the bottom of the recess and the prism holder may alter after a certain period of use. The properties are affected by temperature variations, water/moisture and other environmental factors. Furthermore, the flexibility can be inhibited over time, or an undesired flexibility in other directions may develop because of the ingress of foreign elements, or because of structural changes in the recess in the surface in which the device is installed.
The first and second disadvantages are both related to the fact that each prism grating in the known device withstands only a highly limited impact force from above. This is due to the fact that the prism grating is secured in place in the prism holder by means of an adhesive without any appreciable support against a lower supporting structure. Thus, one of the purposes of the elastic flexibility in the vertical direction is to prevent damage to or loosening of the prism gratings.
A third disadvantage of the known device is that there are difficulties in gaining access to the internal components, for example, when changing lamps or carrying out other maintenance operations.
A fourth disadvantage of the known device is that there are difficulties in removing or replacing the prism gratings as they are permanently bonded to the prism holder by means of adhesive.
A fifth disadvantage of the known device is that the light sources are not utilized in a sufficiently effective manner, as each light source is placed at a relatively great distance from the end of the associated prism grating facing the light source.
A sixth disadvantage of the known device is that the disclosed angle of 45° between the principal direction of the light source and a vertical axis does not give an optimum angle of reflection to ensure desired visibility from certain distances from the device and heights above the plane in which the device is to be installed.
A seventh disadvantage of the known device is that the prism holder and the prism gratings form a relatively large hollow space, where temperature gradients and temperature changes can cause condensation.
An eighth disadvantage of the known device is that the useful life of the light sources is shorter than desired, which is due in part to the fact that the heat generated by the light sources is not conducted away in a satisfactory manner, and causes a higher operating temperature than the optimum.